Can you use two strands of double knit for chunky?

Yes. It isn't the number of plies that determine whether a yarn is called DK (double knitting weight), but the over all thickness of the yarn. Plus, you don't necessarily have to use DK yarn to double knit. Any two yarns of matching thickness can be used. You should check your pattern for the correct gauge and weight of yarn to make sure the finished garment will fit as expected.